I hope I don't step on any toes here, but here goes...
A perfect soundproof room is 'a room within a room' and is built on an isolating material (rubber blocking for instance) to stop sound traveling to the existing structure. Then new walls are fitted to the new (floating) floor, and ceiling joists fitted to them (but not touching the existing ceiling above). This is normaly not practicable, so just new walls are built.
To make a pukka job of soundproofing, ideally you would make a stud wall using 4 x 2 timber in the same way as you would a normal partition wall. Sound transmission thru floor and ceiling to the walls is not as great as you might think, so fitting the studd to a new horizontal 4 x 2 top and bottom timber is OK provided it doesn't touch the wall it is being built in front of.
Make sure this studd wall is one inch away from the existing wall. Vibrations/sound will travel to what it touches.
Fill the studd wall with 4ins of insulation material (find out how much better the acoustic stuff is over the stuff you'd get from Wickes if it's a lot dearer, it's probably not much).
Fit two layers of plasterboard using two 1/2inch or one 1/2inch and one 5/8inch. Use no-more-nails or similar between layers. Use the acoustic matting between for extra mass if you want (more mass the better i would say, but I'd find out more info first). Seal up all joins in the first layer using high modulus silicon rubber or similar.
Plaster as normal to finish.
Seal all edges and joins etc with silicon. Where air can travel, so can sound. Then fit your skirting and seal again.
DON'T use resiliant channel. Where this is good for sound isolation, it can also kill bass waves dead, so no matter how many subs you fit, the flex in the walls will cancel out the bass waves. That's why a 4x2 frame is better tha a 2x1 frame. You may get away with it if you fit them and your room *may* sound nice and bassy, but if it doesn't, you'll either have to live with it or remove them. failing that, fit bass shakers to the seating!
The insulation kills the higher frequencies, and only mass can stop bass wave travel, hence the two layers of plasterboard (and matting if you use it).
For more info try
www.avsforum.com in the 'theater construction' forum. Dennis Erskine does this for a living and he certainly knows his job. Use the search facility and treat his comments as gospel.
If you can't afford to lose 6ins from the wall (1inch gap, 4inch wood, 1inch plasterboard) then use normal studding. It's not as rigid but will do the job to a lesser degree. It's what I did in my loft and is effective. See my homepage for construction pics. I did mine myself using materials from B&Q and Wickes.
My weak link is the cable duct in my rear wall which is insulated but still an air path for sound into the roof space.
Having said that, when doing a basic 'sound test' with my g/f outside in the front and back gardens (not at the same time
), she couldn't hear anything at all, even though I turned the volume up to trebble the normal setting. I was in the loft at the time and it was deafening.
I have a spl and could do a sound test, but ideally this would be at 2am when the ambient is at its least.
I will say that my loft soundproofing is a compromise, but is more effective than I thought it would be. Bass sound is pretty good, but then I have some 'shakers in the seats for added effect.
HTH
Gary.
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